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There seems to be a growing excitement about the “Supermoon” that is due to occur on 14 November 2016, when the Moon will be at its closest to Earth in this orbit, and closer than it has been at any time since 1948.

Sure the full moon will look big and bright this Sunday evening/Monday morning, but no more so than normal. It’ll be 14% larger and 30% brighter but your eyes and brain won’t see any difference from a normal full moon. It’s still worth a look – the moon is always a beautiful thing to see – but if you notice any difference in size or brightness, it’ll be your brain playing tricks on you. Maybe the power of suggestion or the Moon Illusion.

Read on to find out why it’s not any more super than normal…

The Moon orbits the Earth in an elliptical orbit, i.e. it is not perfectly circular, and so in each orbit there is a closest approach, called “perigee” and a furthest approach, called “apogee”.

At this month’s perigee the Moon will be 356,511km away from Earth. Closer than normal, sure but only 66km closer than the “Supermoon” that began all the hype back in 2011, so not that much closer.

Let’s start by comparing it to the Moon’s average distance from the Earth, which is ~385,000km. This perigee will be ~8% closer to the Earth than average. OK, that’s a bit closer, but not significantly so.

What about comparing it to the Moon’s average perigee distance, which is ~364,000km. So this “Supermoon” will be ~2% closer to the Earth than it is most months at perigee. Wow!

The brilliant XKCD sums up the Supermoon hype nicely

So what will this mean to you? Nothing at all. The Moon will be 14% bigger in the sky, but your eye won’t really be able to tell the difference. It will also be 30% percent brighter, but your eye will compensate for this too, so altogether this “Supermoon” will look exactly the same as it always does when it’s full.

As to all of those soothsayers claiming that there will be earthquakes and tidal waves. There very well might be, but they’ll be nothing at all to do with the Moon.

Post Script

Supermoons aren’t all that rare. In fact they occur once every 13.5 months.

Thanks to Steve Bell at the UK Hydrographic Office for providing the calculations below:

The Moon orbits the Earth once every 27.321 days (called the sidereal period), but as the Earth is orbiting the Sun at the same time, the Moon’s phases appear to repeat every 29.530 days (called the synodic period, which is the time we use to derive the month).

The Moon’s orbit is elliptical (a squashed circle) and so you would expect a perigee once every 27.321 days. However the elliptical path around which the Moon orbits the Earth precesses (that is it is not fixed with the perigee occurring at the same part of each orbit; the place where perigee occurs moves, or precesses) with a period of 8.8504 years, so that perigee doesn’t occur once every 27.321 days but rather once every 27.554 days (called the anomalistic period).

To calculate the frequency of perigee full moons (“Supermoons”) you need to use the equation:

1/P(perigee&full) = 1/P(perigee) – 1/P(full)

where P(perigee) is the anomalistic period = 27.554 days, and
where P(full) is the synodic period = 29.530 days

and when you put those figures in you find that a full moon will occur at perigee once every 411.776 days (i.e. P(perigee&full)=411.776), or just less than once per year.

You might have seen that there will be a Full Moon this Christmas Day, this first time this has happened since 1977.

It’s been described on social media as the “perfect” Full Moon. Quite what that means I’m not sure, but there is one factor that makes this month’s Full Moon stand out against the others; it will be this year’s highest Full Moon. But this is true of the Full Moon that happens nearest the winter solstice every year, so there’s nothing particularly “perfect” about this one.

A Full Moon occurs when the Moon is opposite the Sun in our sky, and we see the entire lit hemisphere of the Moon, making it appear full and round.

After it’s full, the Moon will appear to shrink to a gibbous moon (less than full but more the half), a half moon (called the last quarter moon), then a crescent, then a new moon, before growing again through crescent, half, gibbous and back to full. This pattern repeats itself every 29 days, which is approximately one month.

But because it isn’t exactly a month, and because our months vary in length between 28 and 31 days, the Full Moon doesn’t occur on the same numbered day every month, it drifts, seemingly at random.

For example the last decade’s December Full Moons happened on 15, 5, 24, 12, 2 AND 31*, 21, 12, 28, 17, 6 December.

(* In 2009 there were two full moons in December; the second is colloquially known as a blue moon.)

As the 29-day pattern of phases drifts around the 31 days in December it’s not surprising that the occurrence of Christmas Day full moons is around once every thirty years.

The last time this happened was 38 years ago in 1977; the next time it will happen is 19 years in the future, in 2034; an average of every 29 years.

Observing the Moon

If Santa was generous enough to bring you a telescope for Christmas, the Moon is one of the very best places to start. But you don’t want to observe it when it’s full.

This is mainly due to the overall lack of contrast and shadow on the Moon’s surface when it’s lit head-on by the Sun.

The best views of the Moon can be had when you can clearly see the dividing line between light and dark. This line is called the terminator, and when you observe it through a telescope the Moon appears in 3D, with shadows inside craters and beside mountains that really give you a feel for the structure of the Moon’s surface.

In addition the Full Moon is so bright that it drowns out the light of lots of faint astronomical objects, meaning that you would ideally wait till the moon is New, or a think crescent, before venturing out with your telescope to hunt for galaxies and nebulae.

So for stargazers this Christmas Day Full Moon is very far from “perfect”…